Optimized and self optimized magnetic confinement
Friedrich Wagner (Max-Planck-Institut für Plasmaphysik, Teilinstitut Greifswald)
Magnetic confinement in toroidal fusion systems has been developed to a degree, which allows building the first fusion reactor, ITER. The goal of ITER is the operation of a Deuterium-Tritium plasma under conditions, which allow the production of 500 MW fusion power with a power amplification of Q=10. But ITER is still an experiment and it will solve for its basic concept – the tokamak – many fundamental research issues. As the tokamak concept has drawbacks – prone to current driven instabilities and intrinsically pulsed – alternatives are pursued in parallel. The stellarator is the second most advanced toroidal magnetic confinement concept whereat the largest and most advanced device of this line, Wendelstein 7-X, has just started operation in Greifswald, Germany. Wendelstein 7-X is an experiment for basic research, which nevertheless should demonstrate the reactor viability of this concept. The major difference between tokamak and stellarator is the way the poloidal field, essential for confinement, is produced. In case of the tokamak, it is generated by a strong toroidal current induced within the toroidal plasma ring. In stellarators, it is produced externally by currents within strongly shaped coils.
The system optimization aims i.a. at the quality of the thermal insulation of the plasma to reach high core temperatures by economic means. The tokamak benefits from the reduced system symmetry with continuous symmetry in toroidal direction. The stellarator plasma is unavoidably 3-dimensional and toroidally periodic with the corollary of insufficient collisional confinement. But with a rigorous system optimization those field properties which are decisive for the “laminar” radial transport can be made 2-dimensional. Systems with these features are called quasi-symmetric and they should have similar collisional transport like truly symmetric systems, be more stable and intrinsically suitable for steady-state operation.
On the other hand, fusion plasmas are thermodynamically open systems where turbulent processes govern radial transport in symmetric and quasi-symmetric systems. Like in other open systems also, self-organized processes can determine important and relevant plasma characteristics. Under high plasma pressure giving rise to a large turbulence level, the turbulent eddies can develop a coherence where ultimately the energy of the turbulence field is transformed into the kinetic energy of a macroscopic flow. This flow turns out to be sheared acting back onto its original drive, the turbulence field. Within a sub-millisecond time scale, the turbulence is quenched in a highly non-linear process and a quiescent regime with good confinement emerges. This regime is called high-confinement mode (H-mode) and is the basis to reach the goals of ITER. Some of these mechanisms play also a role in other open systems with classical turbulence.
In this seminar the physics of plasma optimization and self-optimization will be presented.